Method for grain refining of steel, grain refining alloy for steel and method for producing grain refining alloy

ABSTRACT

The present invention relates to a method for grain refining of steel. A grain refining alloy having a composition FeXY where X is one or more elements selected from the group consisting of Cr, Mn, Si, Ni, and Mo and where Y is one or more oxide and/or sulphide and or nitride and/or carbide forming elements selected from the group consisting of Ce, La, Nd, Pr, Ti, Al, Zr, Ca, Ba, Sr, Mg, C and N where X is between 0.001 and 99% by weight based on the weight of the alloy and where Y is between 0.001 and 50% by weight of the alloy, said alloy additionally containing between 0.001 and 2% by weight of oxygen and/or between 0.001 and 2% by weight of sulphur, said alloy containing at least 10 3  inclusion particles per mm 3  consisting of oxides and/or sulphides and/or carbides and/or nitrides of one or more of the Y elements and/or one or more of the X elements Cr, Mn and Si in addition to Fe, said inclusion particles having a mean diameter of less than 10 μm, is added to molten steel in an amount of between 0.01 and 5% by weight based on the weight of the steel, whereafter the steel is cast. The invention further relates to a grain refining alloy for steel and to a method for producing grain refining alloys.

FIELD OF INVENTION

The present invention relates to a method for grain refining of steel,particularly ferritic and austenitic steels, a grain refining alloy forsteel and to a method for producing a grain refining alloy. The alloy isdesigned for grain size control of shaped castings and slabs for furtherworking to standard stocks (i.e. sheet, plate, tube, bar, wire or rod).

BACKGROUND ART

The demand for higher performance steels with optimal combination ofproperties is becoming more crucial. Since the grain size in steelcontrols the resulting properties, the desired property profile can beobtained by development of a properly adjusted microstructure.

As-cast steels are prime examples of materials where the propertiesachieved depend upon the characteristics of the solidificationmicrostructure. In general, a coarse columnar grain structure willinevitably evolve upon solidification if potent heterogeneous nucleationsites ahead of the solidifying front are absent. In the presence ofeffective seed crystals, fine equiaxed grains form directly in the melt.Depending upon the circumstances, the equiaxed grain structure maycompletely override the inherent columnar grain formation, which, inturn, gives rise to an improved castability (e.g. hot ductility and hotcracking resistance) through a smaller grain size and reduced problemswith centre-line segregation.

Experience has shown that the as-cast microstructures of high alloyedsteels are quite different from those of the pure carbon manganese orlow alloy steels due to their higher alloy content and broader span inchemical composition. Four distinct solidification modes are commonlyobserved:

-   Primary ferrite formation-   Primary ferrite formation followed by a peritectic transformation to    austenite-   Primary ferrite and austenite formation-   Primary austenite formation

Due to the absence of subsequent solid state phase transformations,there is particularly a need of grain refining in fully austenitic orferritic steels. At present, no grain refiners are commerciallyavailable for steels, as opposed to cast iron and aluminum alloys wheresuch remedies are widely used to refine the solidificationmicrostructure.

Over the past decades, significant improvement of steel properties hasbeen achieved through strict control of the chemical composition, volumefraction and size distribution of non-metallic inclusions. This has beenmade possible by the introduction of secondary steelmaking as anintegrated step in the production route and the use of advanced ladlerefining techniques for deoxidation and desulphurisation. Thedetrimental effect of inclusions on steel properties arises from theirability to act as initiation sites for microvoids and cleavage cracksduring service. Hence, the use of clean steels is normally considered tobe an advantage, both from a toughness and a fatigue point of view.

More recently, the beneficial effect of inclusions on the solid statetransformation behaviour of steels has been highlighted and recognised.In particular, the phenomenon of intragranular nucleation of acicularferrite at inclusions is well-documented in low alloy steel weld metals,where the best properties are achieved at elevated oxygen and sulphurlevels owning to the development of a more fine-grained microstructure.The same observations have also been made in wrought steel productsdeoxidised with titanium, although the conditions existing insteelmaking are more challenging due to the risk of inclusion coarseningand entrapment of large particles that can act as initiation sites forcleavage cracks. Because of the problems related to control of theinclusion size distribution during deoxidation and casting, the conceptof inclusion-stimulated ferrite nucleation have not yet found a wideapplication, but is currently limited to certain wrought steel productswhere the weldability is of particular concern.

Inclusions are known to play an important role in development of thesteel solidification microstructure and substantial grain refining hasbeen observed in a number of systems, including

Aluminum-titanium deoxidised low alloy steels due to nucleation of deltaferrite at titanium oxide/nitride inclusions.

Aluminum-titanium deoxidised ferritic stainless steels due to nucleationof delta ferrite at titanium oxide/nitride containing inclusions.

Rare earth metal (REM) treated low alloy steels due to nucleation ofdelta ferrite at Ce/La containing oxides and sulphides.

Rare earth metal (REM) treated ferritic stainless steels due tonucleation of delta ferrite at Ce/La containing oxides and sulphides.

Rare earth metal (REM) treated austenitic stainless steels due tonucleation of austenite at Ce/La containing oxides and sulphides.

In all cases the grain refining effect is related to the ability of theinclusions to act as efficient heterogeneous nucleation sites, e.g. byproviding a low lattice disregistry between the substrate and thenucleus. Experiments have shown that the undercooling required totrigger a nucleation event is of the order of 1° C. when the atomicmisfit across the interface is 5% or lower. This degree of undercoolingis sufficiently low to promote the formation of an equiaxedmicrostructure during solidification, provided that number density ofthe nucleating inclusions ahead of the advancing solid/liquid interfaceexceeds a certain threshold.

FeSi-based inoculants and treatment alloys for cast iron arecommercially available and commonly used in the foundry industry. Thesealloys contain balanced additions of strong oxide and sulphide formerssuch as Ca, Al, Ce, La, Ba, Sr or Mg. It is well established that themajor role of the minor elements is to modify the chemical compositionand crystal structure of the existing inclusions in the liquid iron,thus promoting the graphite formation during solidification. This occursby a process of heterogeneous nucleation analogous to that documentedfor grain nucleation in steel.

Experiments have shown that both low carbon (LC) FeCr and FeMn, producedby means of conventional casting methods, contain an intrinsicdistribution of oxides and sulphides, the former group being mostimportant. These systems have a high oxygen solubility in the liquidstate (about 0.5% O by weight or higher), where the inclusions formnaturally both prior to and during the casting operation owing toreactions between O and S and Cr, Si and Mn contained in the alloys.However, because the cooling rate associated with conventional sandmould casting is low, the resulting size distribution of the Cr₂O₃,SiO₂, MnO or MnS oxide and sulphide inclusions is rather coarse.Typically, the size of the inclusions in commercial LC FeCr and FeMn isbetween 10 and 50 μm, which make such alloys unsuitable for grainrefining of steel.

Controlled laboratory experiments have shown that the additions of astrong oxide and sulphide former such as Ce to a liquid ferrous alloywill result in the formation of Ce₂O₃ and CeS. These inclusions aresimilar to those observed in steels treated with rare earth metals, andin both cases extensive grain refinement is achieved. The initial sizeof the inclusions obtained with this conventional alloying technique isbetween 1 and 4 μm. However coarsening of the inclusion populationoccurs gradually with time after the Ce addition, and unless the melt israpidly quenched thereafter the inclusions will grow large andeventually become detrimental to mechanical properties. Thus, the realchallenge is either to create or introduce small non-metallic inclusionsin the liquid steel that can act as heterogeneous nucleation sites fordifferent types of microstructures during solidification and in thesolid state (e.g. ferrite or austenite), without compromising theresulting ductility or fracture toughness. In practice, this can beachieved by the use of a novel alloying technique, based on additions oftailor made grain refining alloys to liquid steel where the necessaryreactants or seed crystals are embedded.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a method for grainrefining of steels, particularly ferritic and austenitic steels, byadding certain grain refining alloys to molten steel before or duringcasting, to provide a grain refining alloy for the use in grain refiningof such steels and to provide a method for production of a grainrefining alloy.

Thus according to a first aspect, the present invention relates to amethod for grain refining of steel, particularly ferritic and austeniticsteels, which method is characterised in that a grain refining alloyhaving a composition FeXY where X is one or more elements selected fromthe group consisting of Cr, Mn, Si, Ni and Mo, and where Y is one ormore oxide and/or sulphide and/or nitride and/or carbide formingelements selected from the group consisting of Ce, La, Nd, Pr, Ti, Al,Zr, Ca, Ba, Sr, Mg, C and N where X is between 0.001 and 99% by weightbased on the weight of the alloy and where Y is between 0.001 and 50% byweight of the alloy and said alloy additionally containing between 0.001and 2% by weight of oxygen and/or between 0.001 and 2% by weight ofsulphur, said alloy containing at least 10³ inclusion particles per mm³consisting of oxides and/or sulphides and/or carbides and/or nitrides ofone or more of the Y elements and/or one or more of the X elements Cr,Mn and Si in addition to Fe, said inclusion particles having a meandiameter of less than 10 μm, is added to molten steel in an amount ofbetween 0.01 and 5% by weight based on the weight of the steel,whereafter the steel is cast.

According to a preferred embodiment the FeXY alloy added to the moltensteel contains at least 1% by weight of the X elements.

According to another preferred embodiment the FeXY alloy added to themolten steel contains between 5 to 50% by weight of Fe, 20 to 94% byweight of the X elements, and 0.01 to 30% by weight of the Y elements.The content of oxygen and/or sulphur is preferably between 0.01 and 1%by weight based on the weight of the alloy.

According to another preferred embodiment the grain refining alloy addedto the molten steel contains at least 10⁵ inclusion particles per mm³where the said inclusion has a mean diameter of less than 2 μm.

According to yet another preferred embodiment the grain refining alloyis added to molten steel in an amount between 0.1 and 1.5% by weightbased on the weight of steel.

In order to obtain the best results; that is either to create orintroduce small inclusions in the steel melt that can act asheterogeneous nucleation sites for different microstructures duringsubsequent steel processing, it is important that the liquid steel isproperly treated using one or more grain refining alloys that are addedin succession in order to maximise the grain refining effect. The grainrefining alloy is therefore added to the molten steel either in theladle, in tundish and/or just before or during casting, or directly inthe casting mould following primary steel deoxidation. When the grainrefining alloy is added to the molten steel in the ladle or the tundishit is preferred to add the alloy in the form of a particulate alloyhaving a particle size between 0.2 and 20 mm, preferably between 0.5 and5 mm. When a grain refining alloy containing a fine distribution ofsmall inclusions according to the present invention is added to themolten steel in the casting mould it is preferred to add the alloy inthe form of a cored wire which is continuously added to the steel at acontrolled rate. In order to maximise the grain refining effect thecored wire addition should be made as the last step before casting; thatis after any adjustment of the steel composition and after otherpreconditioning step like deoxidation, previous grain refining alloyadditions or the like.

It has been verified that by the method of the present invention a highnumber of very small oxide and/or sulphide and/or nitride and/or carbideinclusion particles are either created or introduced in the moltensteel. These inclusion particles can act as active nucleation sitesduring the subsequent solidification on which the new grains will growepitaxially until they impinge on and obstruct the growth of thecolumnar grains. This results in a wider equiaxed zone with a smallergrain size and/or a shorter dendrite arm spacing in the as-cast steel.It has also been found that the inclusion particles either created orintroduced in the steel via the grain refining alloys influence themicrostructure evolution in the solid state by affecting the steelrecrystallisation and grain growth behaviour and/or by promotingintragranular nucleation of ferrite or austenite. This leads toadditional grain refining during thermomechanical processing and weldingof the steels, particularly in grades that undergo a subsequenttransformation in the solid state.

According to a second aspect, the present invention relates to an alloyfor grain refining of steel, particularly ferritic and austeniticsteels, characterised in that the grain refining alloy has a compositionFeXY where X is one or more elements selected from the group consistingof Cr, Mn, Si, Ni, and Mo, and where Y is one or more oxide and/orsulphide and/or nitride and/or carbide forming elements selected fromthe group consisting of Ce, La, Nd, Pr, Ti, Al, Zr, Ca, Ba, Sr, Mg, Cand N where X is between 0.001 and 99% by weight based on the weight ofthe alloy and where Y is between 0.001 and 50% by weight of the alloyand said alloy additionally containing between 0.001 and 2% by weight ofoxygen and/or between 0.001 and 2% by weight of sulphur, said alloycontaining at least 10³ inclusion particles per mm³ consisting of oxidesand/or sulphides and/or carbides and/or nitrides of one ore more of theY elements and/or one or more of the X elements Cr, Mn and Si inaddition to Fe, said inclusion particles having a mean diameter of lessthan 10 μm.

According to a preferred embodiment the grain refining alloy contains atleast 1% by weight of the X elements.

According to another preferred embodiment the grain refining alloycontains between 5 to 50% by weight of Fe, 20 to 94% by weight of the Xelements and 0.01 to 30% by weight of the Y elements. The content ofoxygen and/or sulphur is preferably between 0.01 and 1% by weight basedon the weight of the alloy.

According to another preferred embodiment the grain refining alloycontains at least 10⁵ inclusion particles per mm³ where the saidinclusion particles have a mean diameter of less than 2 μm.

The grain refining alloy according to the present invention containingthe desired constituent elements and inclusion size distribution iscrushed and screened to a particle size between 0.2 and 20 mm before itis used as a grain refiner. The grain refining alloy is either added tothe steel in a particulate form or in the form of a cored wire where thecored wire containing the grain refining alloy is produced inconventional way. The cored wire contains a properly adjusted sizedistribution of the crushed particles to obtain the desired packingdensity and dissolution characteristics for a late addition in thecasting mould. If desirable, sulphide and/or oxide-containing compoundscan be mechanically or chemically mixed with the crushed grain refinerand added to the liquid steel via the cored wire.

By selecting a proper combination of X and Y elements in the grainrefining alloy and exercising strict control of the inclusioncomposition, number density and size distribution, the grain refiningalloy can be tailor made for any steel composition. Thus the grainrefining alloy of the present invention is very flexible and can,particularly by selecting the X elements in the alloy, be used to obtaina grain refined steel having the correct amount of alloying elements fora particular steel.

According to a third aspect, the present invention relates to a methodfor producing a grain refining alloy for steel, said method beingcharacterised by the following steps:

providing a molten FeX alloy where X is one or more elements selectedfrom the group consisting of Cr, Mn, Si, Ni, and Mo in an amount between0.001 and 99% by weight of the FeX alloy, the reminder except forimpurities being Fe;

providing a FeXY alloy in molten or solid, particulate state where X isone or more elements selected from the group consisting of Cr, Mn, Si,Ni, and Mo in an amount between 0.001 and 99% by weight of the FeXYalloy and where Y is one or more elements selected from the groupconsisting of Ce, La, Nd, Pr, Ti, Al, Zr, Ca, Ba, Sr, Mg, C and N in anamount of between 0.001 and 90% by weight of the FeXY alloy;

optionally adding an oxide and/or a sulphur-containing compound to themolten FeX alloy to obtain between 0.002 and 4% by weight of O and/orbetween 0.002 and 4% by weight of S dissolved in the molten alloy;

mixing the molten FeX alloy and the molten or solid FeXY alloy in suchamounts that it is obtained a resulting molten alloy consisting of 0.001to 99% by weight of one or more elements selected from the groupconsisting of Fe, Cr, Mn, Si, Ni and Mo, 0.001 to 50% by weight of oneor more elements selected from the group consisting of Ce, La, Nd, Pr,Ti, Al, Zr, Ca, Ba Sr, Mg, C and N, 0.001 to 2% by weight of O and/or0.001 to 2% by weight of S, the reminder being impurity elements, and;

solidifying the resultant molten alloy by casting or quenching to form asolid alloy having at least 10³ inclusion particles pr. mm³ consistingof oxides and/or sulphides and/or carbides and/or nitrides or one ormore of the Y elements and/or one or more of the X elements Cr, Mn andSi in addition to Fe, said inclusion particles having a mean diameter ofless than 10 μm.

According to a preferred embodiment the molten FeX alloy and the moltenFeXY alloy are heated to a temperature of at least 50° C. above theirmelting points before the molten FeX alloy and the molten FeXY alloy aremixed.

According to another embodiment the molten FeX alloy is heated to atemperature of at least 50° C. above its melting point before the solid,particulate FeXY alloy is mixed with the molten FeX alloy.

According to another preferred embodiment the mixing of the molten FeXalloy and the molten FeXY alloy is done by pouring the two meltssimultaneously in such a way that the two melts are brought intointimate contact with each other.

According to yet another embodiment the pouring and mixing of the twomelts are carried out inside a closed chamber.

According to yet another embodiment the resulting molten alloy isimmediately after mixing of the two melts transferred to a separateholding ladle to promote slag/metal phase separation and for removal ofany large inclusions before the melt is being cast or quenched.

The casting or quenching can either be done using a mould, awater-cooled copper chill, or a casting belt, by water granulation, bywater atomization, by gas atomization or by other conventional fastquenching media.

Experience has shown that it is possible to obtain a fine distributionof oxides and/or sulphides and/or nitrides and/or carbides containingthe Y elements and/or one or more of the X elements Cr, Mn and Si inaddition to Fe by controlling the cooling rate of the alloy prior to andduring solidification. Thus by the use of an appropriate melt mixing,casting and/or quenching procedure it is possible to obtain as many as10⁷ inclusion particles per mm³ or higher in the grain refining alloyproduced according to the present invention.

Some embodiments of the present invention will now be further describedby way of examples.

EXAMPLE 1 Manufacturing Of Grain Refining Alloys

Two different grain refining alloys were manufactured according to themethod of the present invention.

Grain Refiner 1

A first grain refining alloy called Grain Refiner 1 was produced asfollows:

A Fe—Cr base alloy containing approximately 65% by weight Cr, 0.05% byweight C, 0.5% by weight Si and 0.01% by weight S was melted in aninduction furnace, using a MgO crucible. The melt was superheated toabout 1700° C., which is approximately 50° C. above the liquidustemperature of the alloy. A silicon-rich and a cerium-rich source werethen added in a particulate form to this melt in succession in order toobtain a new liquid Fe—Cr—Si—Ce alloy. This alloy was subsequentlyquenched in a graphite mould, crushed and screened to yield a particlesize between 0.5 and 4 mm. An analysis of the screened material gave thefollowing result: 31.9% by weight Cr, 15.8% by weight Si, 8.5% by weightCe, 1.18% by weight C, 0.37% by weight O and 0.002% by weight S, theremainder being Fe and other impurity elements. Moreover, subsequentoptical and scanning electron microscope (SEM) examinations of thescreened material revealed evidence of a duplex microstructureconsisting of one Cr, Si and Fe-rich phase and one Ce, Si and Fe-richphase. At the same time faceted Si, Mg and Al-containing non-metallicinclusions were present in the matrix with a mean size of about 5 μm anda local number density being higher than 10³ particles per mm³.

Grain Refiner 2

A second grain refining alloy called Grain Refiner 2 was producedaccording to the following procedure:

A Fe—Cr base alloy containing approximately 65% by weight Cr, 0.05% byweight C, 0.5% by weight Si and 0.01% by weight S was melted in aninduction furnace, using a MgO crucible. The melt was superheated toabout 1700° C., which is approximately 50° C. above the liquidustemperature of the alloy. Iron oxide in a particulate form was thenadded to this liquid melt to achieve oxygen saturation and incipientchromium oxide formation. A second Fe—Cr—Si—Ce alloy was melted inparallel in another induction furnace. The second alloy was superheatedto a temperature of more than 100° C. above the liquidus temperature ofthe alloy. The two melts were subsequently mixed by pouring the liquidFe—Cr—Si—Ce alloy into the liquid oxygen-saturated Fe—Cr alloy. Aftermixing, the resulting molten ally was quenched in a graphite mould,crushed and screened to yield a particle size between 0.25 and 2 mm. Ananalysis of the solid bottom part of the as-cast material gave thefollowing result: 52.7% by weight Cr, 6.7% by weight Si, 0.85% by weightCe, 0.66% by weight C and 0.05% by weight O, the remainder being Fe andother impurity elements. Moreover, subsequent optical scanning electronmicroscope (SEM) examinations of the produced grain refining alloyrevealed evidence of both TiN and Ce-rich faceted inclusions embedded inthe matrix with a mean size less than 2 μm and a number density whichlocally exceeded 10⁷ particles per mm³. Thus, by using two melts, onesaturated with oxygen and one containing the reactive elements, mixingthe melts and quenching the mixed melt, it is possible to tailor-makethe grain refining alloy as to chemical composition, crystal structure,size distribution and number density of the inclusion particles.

EXAMPLE 2 Grain Refining Of Steel

The steels used in the grain refining of steel in this example 2approximately comply with the duplex (austenite-ferrite) variant AISI329 (or DIN 1.4460), having the following range in chemical composition;25-28% by weight Cr, 4.5-6.5% by weight Ni, 1.3-2.0% by weight Mo, max2.0% by weight Mn, max 1.0% by weight Si, max 0.03% by weight S, max0.04% by weight P and max 0.1% by weight C. A charge of about 800 kg wasprepared by induction melting of appropriate scrap material, which wassubsequently alloyed with chromium, nickel and molybdenum to achieve theabove target chemical composition. The temperature of the liquid steelwas between 1580 and 1590° C.

Reference Steel Casting (Prior Art)

A reference casting was produced by pouring about 100 kg of liquid steelfrom the induction furnace into a separate holding ladle. During thisoperation 0.5 kg of FeSi was added to the molten steel for deoxidationpurposes. After a short holding period 30 kg of the melt was poured intoa sand mould to produce a shaped casting with the following crosssectional dimensions; height: 25 mm, smallest width: 25 mm, largestwidth: 30 mm. Following solidification and subsequent cooling down toroom temperature, the steel casting was cleaned and then heat treated at1000° C. for 30 minutes in a furnace to better reveal the as-castmicrostructure. An analysis of the steel chemical composition gave thefollowing result; 24.7% by weight Cr, 6.0% by weight Ni, 1.7% by weightMo, 0.90% by weight Mn, 1.11% by weight Si, 0.003% by weight S, 0.024%by weight P, 0.07% by weight C, 0.01% by weight Al, 0.01% by weight Ti,<0.001% by weight Ce, 0.063% by weight N and 0.024% by weight O.Standard metallographic techniques were then employed to reveal theresulting grain structure in the cross section of the casting. Thisprocedure involved cutting, grinding, polishing and etching in Vilella(5 ml HCl+1 g picric acid+100 ml ethanol). Optical microscopeexamination showed evidence of columnar grains at the surface and coarseequiaxed grains in the interior of the casting with a mean grain sizelarger than 2 mm. Moreover, the subsequent examination of the referencesteel in the scanning electron microscope (SEM) showed that theinclusions were manganese silicates containing small amounts of aluminumand sulphur (probably in the form of MnS). The mean size of theseinclusions was 2.9 μm and the estimated inclusion number density wasabout 10⁵ per mm³.

Steel Casting Grain Refined According To The Invention

A steel casting was produced by pouring about 100 kg of liquid steelfrom the induction furnace into a separate holding ladle. During thisoperation, 0.5 kg of FeSi and 1.8 kg of the experimental Grain Refiner 1were added in succession for deoxidation and inclusion engineeringpurposes, respectively. After a short holding period 30 kg of the meltwas poured into a sand mould to produce a shaped casting with thefollowing cross sectional dimensions; height: 25 mm, smallest width: 25mm, largest width: 30 mm. Following solidification and subsequentcooling down to room temperature, the steel casting was cleaned and thenheat treated at 1000° C. for 30 minutes in a furnace to better revealthe as-cast microstructure. A check analysis of the steel chemicalcomposition gave the following result; 24.8% by weight Cr, 5.9% byweight Ni, 1.7% by weight Mo, 0.92% by weight Mn, 1.44% by weight Si,0.002% by weight S, 0.024% by weight P, 0.079% by weight C, 0.01 % byweight Al, 0.015 weight Ti, 0.08% by weight Ce, 0.067% by weight N and0.028% by weight O. Standard metallographic techniques were thenemployed to reveal the resulting grain structure in the cross section ofthe casting. This procedure involved cutting, grinding, polishing andetching in Vilella (5 ml HCl+1 g picric acid+100 ml ethanol). Opticalmicroscope examination showed no evidence of columnar grains close tothe surface and fine equiaxed grains in the interior of the casting witha mean grain size of about 0.4 to 0.5 mm. The largest equiaxed grainsize was about 1 mm. Moreover, the subsequent examination of theexperimental steel in the scanning electron microscope (SEM) showed thatthe inclusions were faceted Ce-based oxides containing small amounts ofsilicon. Some of these inclusions appeared in the form of largeclusters. The mean size of all inclusions was 2.3 μm and the estimatedinclusion number density was about 2×10⁵ per mm³. The presence of theseCe-based oxide inclusions which form in the liquid steel as a result ofthe addition of Grain Refiner 1 creates favourable conditions fornucleation and growth of ferrite during solidification and subsequentcooling in the solid state.

EXAMPLE 3 Grain Refining Of Steel Ingot For Forging Operations

The steels used in these grain refining experiments comply with thefully austenitic stainless steel variant 254 SMO (or DIN 1.4547), havingthe following range in chemical composition; 19.5-20.5% by weight Cr,17.5-18.5% by weight Ni, 6.0-7.0% by weight Mo, max 1.0% by weight Mn,max 0.7% by weight Si, max 0.010% by weight S, max 0.030% by weight Pand max 0.02% by weight C. Two different heats, each consisting of about5 tons of liquid steel, were prepared in an AOD converter using theappropriate charge materials. After transfer to the tapping ladle themelt temperature was about 1590° C.

Reference Steel Ingot (Prior Art)

Solid rods of mischmetal were added to the liquid steel in the tappingladle as the final preconditioning step. Shortly thereafter the steelwas cast in an iron mould, using a conventional assembly for bottompouring. The total weight of the ingot was 3.4 tons and the dimensionswere as follows; height: 2050 mm, upper cross section: 540×540 mm,bottom cross section: 450×450 mm. After filling the mould with liquidsteel exothermic powder was added on the top of the ingot in order tominimize piping. An analysis of the steel chemical composition gave thefollowing result; 20.1% by weight Cr, 17.6% by weight Ni, 6.2% by weightMo, 0.49% by weight Mn, 0.54% by weight Si, 0.001% by weight S, 0.022%by weight P, 0.03% by weight C, 0.01% by weight Al, 0.01 w % by weightTi, 0.01% by weight Ce, 0.005% by weight La, 0.19% by weight N and0.005% by weight O. Following solidification and subsequent cooling downto room temperature, the steel ingot was sectioned about 500 mm from thetop of the casting. Metallographic samples were taken from threedifferent positions in the length direction of the ingot at this height,i.e. surface position, 70 mm from the surface and in the centre.Standard metallographic techniques were then employed to reveal theresulting grain size and the dendrite structure in these positions.Specifically, the procedure involved grinding, polishing and etching inVilella (5 ml HCl+1 g picric acid+100 ml ethanol). Optical microscopeexamination showed no evidence of a chill zone close to the surface ofthe ingot. At a position 70 mm away from the surface coarse, equiaxedgrains with a corresponding coarse dendrite substructure could beobserved. The solidification microstructure became gradually coarsertowards the centre of the ingot. Moreover, the subsequent examination ofthe reference steel in the scanning electron microscope (SEM) showedthat the inclusions were La—Ce-based oxide particles with a mean size ofabout 2.8 μm and an estimated inclusion number density of about 10⁵ permm³.

Steel Ingot Grain Refined According To The Invention

In this case 3.5 kg of the Grain Refiner 1 was added per ton of liquidsteel in the tapping ladle as the final preconditioning step as areplacement of the mischmetal additions. Shortly thereafter the steelwas cast in an iron mould, using a conventional assembly for bottompouring. The total weight of the ingot was 3.4 tons and the dimensionswere as follows; height: 2050 mm, upper cross section: 540×540 mm,bottom cross section: 450×450 mm. After filling the mould with liquidsteel exothermic powder was added on the top of the ingot in order tominimize piping. An analysis of the steel chemical composition gave thefollowing result; 20.2% by weight Cr, 17.7% by weight Ni, 6.1% by weightMo, 0.58% by weight Mn, 0.39% by weight Si, 0.001% by weight S, 0.025%by weight P, 0.02% by weight C, 0.01% by weight Al, 0.01% by weight Ti,0.01% by weight Ce, <0.001% by weight La, 0.21% by weight N and 0.01% byweight O. Following solidification and subsequent cooling down to roomtemperature, the steel ingot was sectioned about 500 mm from the top ofthe casting. Metallographic samples were taken from three differentpositions in the length direction of the ingot at his height, i.e.surface position, 70 mm from the surface and in the centre. Standardmetallographic techniques were then employed to reveal the resultantgrain size and the dendrite structure in these positions. Specifically,the procedure involved grinding, polishing and etching in Vilella (5 mlHCl+1 g picric acid+100 ml ethanol). Optical microscope examinationrevealed an extremely fine grain size within the chill zone, i.e. from0.05 to 0.1 mm on the average, from which the coarser columnar grainsgrew into the interior of the ingot. At a position 70 mm away from thesurface only coarse equiaxed grains were observed. However, each ofthese grains consisted of a very fine-masked network of dendrites, wherethe dendrite arm spacing was approximately a factor of three smallerthan that observed in the reference steel ingot treated with mischmetal.Also in the centre of the casting the grain refining effect wassubstantial compared with the reference ingot, and at this position thedendrite arm spacing was approximately a factor of two smaller in favourof the steel ingot grain refined according to the present invention.Moreover, the subsequent examination of the grain refined steel inscanning electron microscope (SEM) revealed that the inclusions werefaceted Ce—Al based oxide particles with a mean size of about 2.7 μm andan estimated inclusion number density of about 2×10⁵ per mm³. Theobserved change in the solidification microstructure, which is caused bythe addition of the Grain Refiner 1 in replacement of mischmetal, is dueto the formation of faceted Ce—Al based oxide particles in theexperimental steel ingot. These oxide particles provide favourableconditions for nucleation and growth of austenite during solidificationand subsequent cooling in the solid state.

1. Method for grain refining of steel, comprising: providing a grainrefining alloy having a composition FeXY where X is one or more elementsselected from the group consisting of Cr, Mn, Si, Ni and Mo and where Yis one or more oxide and/or sulphide and/or nitride and/or carbideforming elements selected from the group consisting of Ce, La, Nd, Pr,Ti, Al, Zr, Ca, Ba, Sr, Mg, C and N where X is between 0.001 and 99% byweight based on the weight of the alloy and where Y is between 0.001 and50% by weight of the alloy, said alloy additionally containing between0.001 and 2% by weight of oxygen and/or between 0.001 and 2% by weightof sulphur; quenching said alloy in a molten state such that said alloycontains at least 10³ inclusion particles per mm³ consisting of oxidesand/or sulphides and/or nitrides and/or carbides of one or more of the Yelements and/or one or more of the X elements Cr, Mn and Si in additionto Fe, said inclusion particles having a mean diameter of less than 10μm, adding said quenched alloy to molten steel in an amount of between0.01 and 5% by weight based on the weight of the steel; and casting saidsteel.
 2. Method according to claim 1, wherein the FeXY alloy added tothe molten steel contains at least 1% by weight of X elements.
 3. Methodaccording to claim 1, wherein the FeXY alloy added to the molten steelcontains 5 to 50% by weight Fe, 20 to 94% of the X elements, and 0.01and 30% by weight of the Y elements and the content of oxygen and/orsulphur is between 0.01 and 1% by weight based on the weight of thealloy.
 4. Method according to claim 1, wherein the FeXY alloy added tothe molten steel contains at least 10⁵ inclusion particles per mm³ wherethe said inclusion particles have a mean diameter of less than 2 μm. 5.Method according to claim 1, wherein the grain refining alloy is addedto the molten steel in an amount between 0.1 and 1.5% by weight based onthe weight of steel.
 6. Method according to claim 1, wherein the grainrefining alloy is added to the molten steel in a ladle or a tundish justbefore or during casting.
 7. Method according to claim 1, wherein thegrain refining alloy is added to the molten steel in a casting mould. 8.An alloy having a particle size between 0.2 and 20 mm for grain refiningof steel, wherein the alloy has a composition FeXY, where X is one ormore elements selected from the group consisting of Cr, Mn, Si, Ni andMo and where Y is one or more oxide and/or sulphide and/or nitrideand/or carbide forming elements selected from the group consisting ofCe, La, Nd, Pr, Ti, Al, Zr, Ca, Ba, Sr, Mg, C and N, where said alloycontains between 5 and 50% by weight Fe based on the weight of thealloy, where X is between 20 and 94% by weight based on the weight ofthe alloy and where Y is between 1.51 and 30% by weight of the alloy,and said alloy additionally containing between 0.01 and 1% by weight ofoxygen and/or between 0.01 and 1% by weight of sulphur, said alloy beingproduced by quenching said alloy in a molten state such that said alloycontains at least 10³ inclusion particles per mm³ consisting of oxidesand/or sulphides and/or nitrides and/or carbides of one or more of the Yelements and/or one or more of the X elements Cr, Mn and Si in additionto Fe, said inclusion particles having a mean diameter of less than 10μm.
 9. Alloy according to claim 8, wherein the alloy contains at least10⁵ inclusion particles per mm³ where the said inclusion particles havea mean diameter of less than 2 μm.
 10. Method for producing a grainrefining alloy for steel, comprising the following steps: providing amolten FeX alloy where X is one or more elements selected from the groupconsisting of Cr, Mn, Si, Ni and Mo, in an amount between 0.001 and 99%by weight of the base FeX alloy, the reminder except for impuritiesbeing Fe; providing a FeXY alloy in molten or solid, particulate statewhere X is one or more elements selected from the group consisting ofCr, Mn, Si, Ni and Mo in an amount between 0.001 and 99% by weight ofthe FeXY alloy and where Y is one or more element selected from thegroup consisting of Ce, La, Nd, Pr, Ti, Al, Zr, Ca, Ba, Sr, Mg, C and Nin an amount between 0.001 and 90% by weight of the FeXY alloy;optionally adding an oxide and/or sulphur or a sulphur-containingcompound to the molten FeX alloy to obtain between 0.002 and 4.0% byweight of O and/or between 0.002 and 4.0% by weight of S dissolved inthe molten alloy; mixing the molten FeX alloy and the molten or solidFeXY alloy in such amounts that it is obtained a resulting molten alloyconsisting of 0.001 to 99% by weight of one or more elements selectedfrom the group consisting of Cr, Mn, Si, Ni and Mo, 0.001 to 50% byweight of one or more elements selected from the group consisting of Ce,La, Nd, Pr, Ti, Al, Zr, Ca, Ba, Sr, Mg, C and N, 0.001 to 2% by weightof O and/or 0.001 to 2% by weight of S, the reminder except for normalimpurities being Fe, and; solidifying the resultant molten alloy byquenching to form a solid alloy having at least 10³ inclusion particleper mm³ consisting of oxides and/or sulphides and/or nitrides and/orcarbides of one or more of the Y elements and/or one or more of the Xelements Cr, Mn and Si, in addition to Fe, said inclusion particleshaving a mean diameter of less than 10 μm; and crushing the solid alloyto yield particle having a size between 0.2 mm-20 mm.
 11. Methodaccording to claim 10, wherein the molten FeX alloy and the molten FeXYalloy are heated to a temperature of at least 50 C above their meltingpoints before the molten FeX alloy and the molten FeXY alloy are mixed.12. Method according to claim 10, wherein the molten FeX alloy is heatedto a temperature of at least 50° C. above its melting point before thesolid, particulate FeXY alloy is mixed with the molten FeX alloy. 13.Method according to claim 11, wherein the mixing of the molten FeX alloyand the molten FeXY alloy is done by pouring the two meltssimultaneously in such a way that the two melts are brought intointimate contact with each other.
 14. Method according to claim 13,characterized in that the pouring and mixing of the two melts arecarried out in a closed chamber.
 15. Method according to claim 10,wherein the resulting molten alloy immediately after mixing istransferred to a separate holding ladle to promote slag/metal separationand for removal of any large inclusions before the melt is beingquenched.